High availability (HA) systems have multiple structures. A database management system is duplicated as a high performance solution in order to avoid a complete shutdown due to a power failure or a CPU failure in one database management system. For example, Jim Gray and Andreas Reuter (1993) disclosed hot-standby non-stop operations as a database failure solution based upon the HA system. (Transaction Processing: Concepts and Techniques, Morgan Kaufmann Publishers, 1993). In order to secure high reliability in disk devices, Redundant Array of Inexpensive Disks (RAID) are also combined. Patterson et al. (1988) disclosed the RAID structures in details. (A Case for Redundant Array of Inexpensive Disks, ACM SIGMOD, 1988). Furthermore, a replica database is based upon a common access function for accessing distributed systems. For example, Date (1990) disclosed the above replica database technologies. (An Introduction to Database Systems: Volume 1, Fifth Edition, Addison-Wesley Publisher, 1990).
Despite the above described prior art technologies, it is still necessary to respond to an enormous change in the database amount, the users as well as the data load in the current computing environment. The current environment includes a Web site that is simultaneously accessed by over several tens of thousands of users and or Data WareHouse (DWH) systems that manage large databases with daily addition of over ten TB of analysis data. Although it is possible to initially design a system based upon data division methods, the memory amount, the number of processors and servers, and disk distribution, it is necessary to minimize the initial configuration of the system for the least start-up cost. For this reason, the initial database is redesigned and reconfigured when the database data amount and the number of users increased. To accomplish the redesign and reconfiguration, in the prior art, the operation of the database system is at least temporarily terminated while the data division method, the memory amount, the number of processors and servers as well as the disk storage amount are being reconfigured in accordance with the necessary modifications.
The reason for ceasing the database operations includes a number of factors. For example, in response to ever increasing services and access from clients such as Web browsers, it is necessary to add a server in the database management system to process the above described demands. The database data amount also has ever increased due to increased client service, increased application programs as well as diversified views or reports from the database. Due to these factors, it is necessary to add disk storage units for storing the increased database. It is also necessary to equalize the database access load modifying a schema for data division methods. Furthermore, it is necessary to periodically reconfigure the database in order to prevent search efficiency from declining due to an imbalanced database structure that is caused by certain operations such as the deletion of a large amount of data from the database.
The above described operations for maintenance and or reconfiguration are generally performed while the database system is shut down at night or runs the batch operation during the weekend. However, since database systems on the Web are usually globally accessed for twenty-four hours a day, it is not possible to cease the database operations at night or to run the batch operation. For this reason, it is desired to perform the above described maintenance/reconfiguration in parallel with the database access without interfering the database operations. Furthermore, it is also desired to summarize the statistical trends of the activities with respect to the database while the database system is continuously operated.